The present invention relates to print heads and, in particular, it concerns an arrangement of print heads for reducing distortions in the printed output.
Contemporary high resolution inkjet printers are required to produce resolutions of at least 300 dots per inch (DPI), and typically 600 DPI or greater. However, the construction of the nozzles making up a print head, such as for example a piezoelectric inkjet print head, typically limits the physical proximity between the nozzles to at least one, or even two, orders of magnitude greater than would be required to achieve the required resolution directly.
To overcome this limitation, conventional inkjet printers employ a staggered array to achieve the required resolution. The extent of stagger between the various rows is such that, as the paper moves, the traces of ink drops from the various nozzles define non-overlapping, equally-spaced parallel lines. The spacing of these lines determines the effective resolution of the head. For a 600 DPI inkjet printer, a typical example employs at least 4 staggered rows of nozzles. The timing of the ejection of drops from any one row relative to any other row is made to be equal to the time of paper travel between the rows in question. Thus, for example, in order to print a solid horizontal line at a given vertical position on the paper, each row of nozzles is made to eject an ink drop when the given paper position passes opposite that row.
For reasons of efficient manufacturing and servicing, it is preferable to divide a large single-unit print head into several identical smaller print heads, together forming a print head arrangement. FIG. 1 shows schematically an example of an inkjet print head arrangement 10 of this type having a number of print heads 12a, 12b etc. For simplicity of description, the arrangement is assumed to have 7 print heads, each with 8 staggered rows of nozzles. The seven print heads are staggered relative to each other so as to result in a full 600 DPI print coverage across the width of paper fed in a predefined feed direction.
For clarity of presentation, the structure of FIG. 1 is represented schematically in FIG. 2 by a set of seven staggered identical single-row print heads. The resulting dots timed to fall at the same X-position on the substrate form a printed line 14.
A major shortcoming of this structure is the tendency of the arrangement to cause misregistration in the printed output. As the paper is moved under the head arrangement in direction 16, the dots labeled g and h, which are adjacent in the printed output 14, are generated by nozzles at opposite extreme ends of the arrangement. The long paper travel distance between these end points often gives rise to a slight overlap or gap between adjacent dots forming line 14 due to variations in the paper positioning or paper distortion due to wet paper contraction and different ink drying times. The result is a xe2x80x9cwavyxe2x80x9d rather than straight line output.
There is therefore a need for a print head arrangement which would reduce the distance between dot-generating elements corresponding to adjacent dots in a printed output.
The present invention is a print head arrangement.
According to the teachings of the present invention there is provided, an arrangement of print heads for printing on a substrate moving relative to the arrangement in a given direction of relative motion, the arrangement comprising: a number N of similar print heads where N is at least equal to four, each of the print heads being configured to selectively print a pattern of dots such that relative motion of the print head and the substrate defines a virtual pattern of printable parallel lines, the virtual pattern having a minimum period of repetition D as measured in a direction perpendicular to the direction of relative motion, wherein the print heads are deployed sequentially along the direction of relative motion with a displacement X0 between adjacent ones of the print heads measured parallel to the direction of relative motion, the print heads being offset relative to a given reference position in a direction perpendicular to the direction of relative motion by nY0, where n is an integer value from 0 to (Nxe2x88x921) and Y0=D/N, any two of the print heads which have offsets differing by Y0 or (Nxe2x88x921)Y0 being referred to as xe2x80x9cfunctionally adjacent print headsxe2x80x9d, and wherein the sequence of offsets of the print heads is chosen such that a maximum displacement xcex94Xmax measured parallel to the direction of relative motion between any two functionally adjacent print heads is no greater than (N/2)X0.
Preferably, the sequence of offsets of the print heads is chosen such that the maximum displacement xcex94Xmax measured parallel to the direction of relative motion between any two functionally adjacent print heads is no greater than 2X0.